This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Protein ubiquitination is a post-translational modification widely utilized in eukaryotic regulation. The best known function of ubiquitin is to mark proteins for proteasomal degradation. This and other functions require first activation of ubiquitin by the E1 enzyme, followed by binding to one of ~40 E2 conjugases. Finally, specificity of ubiquitination is conferred by E3 ligases. The activity of most E3s is specified by a RING domain, which binds to the E2~ubiquitin conjugate and activates release of ubiquitin to attack a lysine residue in a substrate protein, bound to a separate domain in the E3. Over 600 human genes encode E3s, consistent with these enzymes'known functions in the control of many cellular processes, and with their involvement in multiple diseases. Solving the structure of this E3 complex will help understand its function and should contribute to the understanding of the mechanism of action of E3s in general. The more detailed our understanding of basic principles underlying E3 function becomes, the more accurately we will be able to understand how they are regulated and what effects disease mutations have on activity. Last but not least, by understanding of the mechanism of action of E3s we should be able to design E3-targeting small molecule inhibitors and interpret the effects of such drugs.